Osteoclast-associated receptor (OSCAR) is a co-stimulatory receptor in osteoclastogenesis. Synovial tissues from active rheumatoid arthritis (RA) patients express higher levels of OSCAR compared with osteoarthritic and normal patients. OSCAR and tartrate-resistant acid phosphatase (TRAP) expression levels did not differ between the cartilage pannus junction (CPJ) and non-CPJ regions in active RA.